1. (Field of the Invention)
The present invention relates to a data transmission system in which a data output is controlled properly when any error or abnormality is detected in the data received.
2. (Disclosure of Prior Art)
Signal transmission systems are used for data transmissions between computer systems, or between a computer and industrial machines, for instance. As systems in generally are highly advanced and become highly reliable, the signal transmission systems are required also to be of higher grade and higher reliability than ever. In these circumstances, the recent signal transmission systems are provided with functions of checking to determine whether data is transmitted without fail or whether there is any abnormality in the data, or the like. When some abnormality is detected in the signal transmission system provided with such abnormality detecting functions as described above, various measures may be taken, such as all output signals may be interrupted forcedly, or the data received just before the detection of the abnormality may be retained, or the state of abnormality may be displayed in a display device, or the like.
It is not preferable, however, to take the measure that all the outputs at the output device, such as the digital output card or analogue output card, are cleared to zero, or that the data received just before detection of some abnormality is retained indiscriminately, when the received data contains an abnormality, because this measure excludes the flexibility at each output device. In other words, there are cases in which a preceding state (data) should be retained at one output device while it should be cleared to zero at another, and it can not be regarded as preferable in these cases that an indiscriminate data control is given to all of the output devices.